Disclosed herein is an invention for the operation of a refrigeration cycle using the strain induced phase change in the crystallographic structure of a material. Of particular interest are certain types of alloys known as shape memory alloys. Like most solids, these materials exhibit a thermoelastic effect. The thermoelastic effect refers to a phenomenon whereby; when a solid is heated or cooled a volume change occurs. Conversely, if a solid""s volume is changed (e.g. by straining it) a temperature change occurs. The thermoelastic effect is well known in the art.
Shape memory alloys, which are known in the art, are able to undergo large plastic strains and then recover this strain when they are heated above a characteristic transformation temperature (Af). This occurs due to a change in phase of the alloy""s grain structure. Above Af the material exists predominately or exclusively in the parent phase (P). Below the temperature Ms, where Ms less than Af, the grain structure is predominately in the martensite phase (M). When the material is in the M-phase it is ductile and deforms easily.
When a shape memory alloy is heated above Af it reverts to the parent phase (P) of the material. If the material was deformed plastically when it was in the M-phase heating to above Af results in the material recovering its pre-strained shape. This is true for even rather large plastic deformations. In addition the forces driving the phase change and hence the shape change are very strong, in fact stronger than the yield strength of the material. Another property associated with most shape memory alloys is that, if a stress is applied to the material when the temperature of the material is above Af the resulting strain results in the growth of the M-phase crystals, just as if the material were at a temperature below Ms. When the stress is removed the strain is recovered and the material reverts to the P-phase. This behavior results in a very springy material that can undergo large recoverable strains as compared to conventional metals. This behavior is termed superelasticity or psuedoelasticity. The temperature Af can be controlled to great precision within a fairly large temperature range when the proper alloying and heat treatments are performed. For example Af can be set at or below room temperature so that the superelastic effect is present at room temperature or below.
The background information above is known in the art. The invention disclosed herein uses the strain induced phase change such as that which occurs in superelastic materials to produce a refrigeration effect. As described below, when operated in a cyclical manner, a refrigeration cycle is produced.
A refrigeration cycle absorbs heat from a low temperature source and rejects it to a high temperature sink. Work input or energy input is required to operate the refrigeration cycle. The refrigeration cycle can also be used to provide efficient heating. This is accomplished by utilizing the heat rejected at the high temperature sink. When operated in this fashion the device is usually termed a heat pump. It should be noted that if the refrigeration cycle is being used for heating or cooling it is the same cycle.
To achieve the refrigeration effect the invention makes use of the fact that the strain induced M-phase in the material also results in an adiabatic and largely reversible temperature increase, of said material. When the strain is removed an adiabatic and largely reversible temperature drop occurs, in said material. In other words the temperature increase observed when the material is strained is, at least in part, not caused by irreversible phenomenon such as friction. By selectively straining the material and rejecting heat to a high temperature sink, and relaxing the material and absorbing heat from a low temperature source, a refrigeration cycle is achieved. The disclosed refrigeration cycle has advantages over other known refrigeration cycles. One advantage is that it is extremely simple and robust. Another advantage is that it does not use chemicals that can deplete the ozone layer or contribute to global warming.
The use of the thermoelastic effect to achieve a refrigeration cycle is known in the art. In particular, it is well documented in the art that elastomers exhibit a thermoelastic effect. For example U.S. Pat. No. 3,036,444 discloses the use of elastomeric blades to achieve a refrigeration effect. What is inventive is using the strain induced phase change of a material, to produce a refrigeration effect. Whereas, in an elastomer, the strain induced temperature change is achieved through partial alignment of the threadlike molecular strands that make up the material, not via a phase change of the material. The present invention has several advantages over elastomer based thermoelastic refrigeration cycles. One such advantage is that unlike elastomers, shape memory alloys have very good fatigue properties. Therefore, a refrigeration device employing shape memory alloys would have substantially greater service life than one using elastomers. Another advantage is that many shape memory alloys, being metallic in nature, are good thermal conductors. This allows for more efficient rejection and absorption of heat, by the heat sink or source respectively.
In a disclosed embodiment of the invention, a coil spring made out of superelastic wire is looped around two pulleys, of unequal diameter. Each pulley is driven at the same rotation rate via drive means. Since the pulleys rotate at the same speed; as the coil spring leaves the small diameter pulley and is pulled onto the large diameter pulley, the spring will stretch. This will increase the temperature of the spring. The spring is now hotter than the ambient air so heat will be lost to the surrounding air and the spring will cool to approximately the same temperature as the surrounding air. A fan or other means of forced air cooling may be used to increase heat transfer. As the wire leaves the large pulley and is pulled onto the small pulley the spring will contract. At the same time the wire enters the refrigerated space. When the spring contracts its temperature decreases to a temperature below that of the refrigerated space. The spring then absorbs heat from the refrigerated space, lowering the temperature of the refrigerated space.
The disclosed embodiment may be modified to use a mesh of superelastic wire or superelastic material in sheet form. In this embodiment drum rollers replace the pulleys. The larger surface area of the mesh or sheet results in increased cooling or heating capacity.
In another embodiment superelastic wire is rolled, in a spiral fashion, along a drum roller. This strain increases the temperature of the wire. Air is forced over the wire while it is on the drum, to enhance the rejection of heat to the surroundings. When the wire unrolls from the pulley the strain is released, and the temperature of the wire drops. At the same time the wire enters the refrigerated space, where it absorbs heat from the refrigerated space.
In yet another embodiment rectangular pieces of superelastic fins are attached to a continuous belt, that is held between two pulleys. The fins face toward the outside of the loop formed by the belt. The fins are attached generally perpendicular to the belt. For a portion of the path traced by the belt the fins come in contact with a guide piece that deforms the fins straining the superelaatic material. The strain increases the temperature of the fins. Cooling air is used to increase heat rejection to the surroundings. When the fins move out of contact with the guide piece they straighten. The removal of strain decreases the temperature of the fins. At the same time the fins enter the refrigerated space where they absorb heat from their surrounds (i.e. the refrigerated space).